Accurately measuring training intensity in swimming remains a challenge due to the limited availability of scientifically validated monitoring devices. The measurement of heart rate (HR), for example, would enable the calculation of HR-based training loads. The FORM Smart Swim 2 goggles measure HR via a temple-mounted optical sensor and enable real-time HR feedback. However, the goggles` HR measurement function has yet to be validated. Twenty-five experienced recreational swimmers simultaneously wore the goggles and a criterion chest-strap (Garmin HRM-Swim) during the completion of an assigned workout. Agreement was assessed at four levels of data aggregation: individual data points, intervals, main-sets, and whole-workouts. Respective mean HR (HRmean) from the goggles and criterion chest-strap were 136 ± 17 bpm and 135 ± 18 bpm (mean absolute percentage error = 1.0 ± 1.2%). For HRmean, concordance correlation was high (intervals: 0.91, 95%CI: 0.87 to 0.94; whole-workouts: 0.99, 95%CI: 0.99 to 1.0), and fixed bias was low (intervals: -0.23 bpm, 95%CI: -1.2 to 0.78 bpm; whole-workouts: -0.24 bpm; 95%CI: -0.94 to 0.46 bpm). Strong agreement was observed between the goggles and criterion chest-strap for HR measurements, particularly for HRmean. Since HRmean serves as the primary input for HR-based training load metrics, the goggles provide valid data for monitoring swim training.
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